Defocuser for compact free space communication

ABSTRACT

Methods, devices, and systems are described for free space optical communication. An example device can comprise a defocuser configured to receive an optical signal from a laser and control a beam divergence of the optical signal. The optical signal can comprise a data signal and a beacon signal. The device can comprise a controller configured to cause the defocuser to adjust the beam divergence based on an operational mode of the laser.

BACKGROUND

Free space optical communication can include communication that useslight propagating in free space to wirelessly transmit data.Conventional devices typically rely on separate optical elements, suchas apertures, lenses, and electronics to manage different phases ofcommunication. This results in devices that are overly bulky and energyinefficient. Thus, there is a need for more sophisticated opticalcommunication techniques.

SUMMARY

Methods, devices, and systems are disclosed for free space opticalcommunication. An example device can comprise a defocuser configured toreceive an optical signal from a laser and control a beam divergence ofthe optical signal. The optical signal can comprise a data signal and abeacon signal. The device can comprise a controller configured to causethe defocuser to adjust the beam divergence based on an operational modeof the laser.

An example method can comprise adjusting, based on a first operationalmode, a defocuser from a first setting associated to a first settingconfigured for a first beam divergence. The method can comprisereceiving, from a laser, a first optical signal. The method can comprisemodifying, using the defocuser adjusted to the first setting, the firstoptical signal to have the first beam divergence. The method cancomprise adjusting, based on a second operational mode, the defocuser toa second setting configured for a second beam divergence. The method cancomprise receiving, from the laser, a second optical signal. The methodcan comprise modifying, using the defocuser adjusted to the secondsetting, the second optical signal to have the second beam divergence.The method can comprise outputting one or more of the modified firstoptical signal or the modified second optical signal.

An example system can comprise a first optical terminal and a secondoptical terminal configured to communicate with the first opticalterminal. One or more of the first optical terminal or the secondoptical terminal can comprise a defocuser configured to receive anoptical signal from a laser and control a beam divergence of the opticalsignal and a controller configured to cause the defocuser to adjust thebeam divergence based on an operational mode of the laser. The opticalsignal can comprise a data signal and a beacon signal.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. Furthermore,the claimed subject matter is not limited to limitations that solve anyor all disadvantages noted in any part of this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate embodiments and together with thedescription, serve to explain the principles of the methods and systems.

FIG. 1 shows an example device in accordance with the presentdisclosure.

FIG. 2A is a block diagram showing a schematic of a transmit portion ofan example device.

FIG. 2B is a block diagram showing a schematic of an example receiveportion of an example device.

FIG. 3 is a graph illustrating an example optical signal transmissionwaveform.

FIG. 4 is a diagram showing an example terminal in accordance with thepresent disclosure.

FIG. 5 is a diagram showing another example terminal in accordance withthe present disclosure.

FIG. 6 shows an example terminal comprising multiple apertures.

FIG. 7 shows a diagram of example optical elements for the secondaperture of FIG. 6.

FIG. 8 is flowchart showing an example method for free space opticalcommunication.

FIG. 9 is a block diagram showing an overall system architecture of anexample device.

FIG. 10 is a diagram showing a simulation of operation of an opticalterminal.

FIG. 11 is diagram showing an example terminal in accordance with thepresent disclosure.

FIG. 12 is a diagram showing an example coarse pointing device.

FIG. 13 is a diagram showing an example base and coarse pointing device.

FIG. 14A shows an example defocuser adjusted for a first beamdivergence.

FIG. 14B shows an example defocuser adjusted for a second beamdivergence.

FIG. 15 is a graph showing beam divergence and lens spacing for anexample defocuser.

FIG. 16A shows another example defocuser.

FIG. 16B shows a graph illustrating how the lens radius of curvature canbe varied based on different voltages to achieve different beamdivergence results.

FIG. 17 is a block diagram illustrating an example computing device.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Disclosed are devices, methods, and systems for optical communication.The disclosed devices can be optimized to minimize size, weight, andpower consumption. An example device can be optimized based one or moreof the following: 1) direct modulation of the laser removing the needfor external optical modulators; 2) a non-amplified transmit signal toremove power inefficiencies from amplification; 3) direct rather thancoherent detection of the signal to remove local oscillator and carrierrecovery requirements; 4) digitization of the received signal using avariable gain limiting amplifier removing the need for a receive ADC; or5) combining PAT (pointing, acquisition, and tracking) and data into asingle optical system and a single optical source.

The disclosed device can comprise optical sources with the 800-900 nmrange, but other wavelength ranges may be used. Aluminum galliumarsenide (AlGaAs) semiconductor sources can be used with wall-plugefficiencies of greater than about 30%. The device can compriselow-noise, high-gain, linear-mode Silicon APDs that provide detectionwith quantum efficiencies of greater than about 80%. While other sourcesnear 980 nm provide similar wall-plug efficiencies, the longerwavelengths have reduced detection efficiency. For example, indiumgallium arsenide (InGaAs) avalanche photodiodes (APDs) can providesimilar detection efficiencies near 1550 nm, but have significantlyhigher noise, and longer wavelength sources have reduced wall-plugefficiencies near 20%. The combination of highly-efficient,high-bandwidth, single-mode sources, and low noise, high-sensitivitydetectors can make the 800-900 nm range an optimal choice for thedisclose device. To increase the potential transmit power and providesystem redundancy, the transponder can utilize two lasers that arepolarization multiplexed together. These lasers can be directlymodulated to provide a NRZ-OOK signal. A receiving portion of the devicecan comprise a linear-mode APD that is first amplified with atransimpedance amplifier and then a limiting amplifier to directlydigitize the received signal without the need for an ADC.

While pulse-position modulation (PPM) theoretically requires fewerphotons-per-bit, its efficiency is only realized when the transmitter isaverage power limited rather than peak power limited. By using directlymodulated semiconductor lasers as a transmit source, the discloseddevices can be peak power limited thus negating the benefits of PPM.While the use of other wavelengths (e.g., 1064 nm, 1550 nm), allow forthe use of optical amplifiers that can provide higher peak power andrealize the benefits of PPM, these other devices are limited by lowerefficiency detectors and sources such that the wall-plug-efficiencyremains low. Coherent detection techniques similarly promise improvedenergy efficiency per bit, but require significant component andprocessing overhead.

To provide efficient data transfer over the wide range of distances from2000-4000 km, the disclosed device can be configured with multi-ratecapabilities in the transponder and automatic selection of the best ratefor a given received signal power. As an example, rates can comprise 64Mbps, 200 Mbps, or any other appropriate rate.

An example device can comprise a lightweight, low power, free-spaceoptical transceiver configured to provide data rates up to at leastabout 600 Mbps, while providing multi-rate capabilities. The link can besymmetric, operating at the full 420 Mbps rate in both directions.Depending on mission needs, the rate can be scalable to an asymmetriclink with a lower total power consumption. However, a symmetric linkdesign saves on both NRE and RE as only a single terminal type needs tobe developed, and provides a lower energy per bit by efficientlyutilizing the overhead required for pointing and tracking. An examplemechanical model is shown in FIG. 1 showing the features of thedisclosed device including a single aperture telescope, articulatingjoint mechanical gimbal, and compact electronics for power conditioning,control, and communications in a single light-weight housing.

The disclosed device can be based on following novel design concepts.The device can be configured to use a communications channel that usesdirect detection and on-off keying with directly modulated semiconductorlasers (e.g., operating at 850/830 nm). Wavelengths can be selected toall for use of semiconductor lasers with greater than about 30%wall-plug-efficiency. High gain avalanche photo diodes (APD) can be usedwith quantum efficiencies of greater than about 80%. Combined with thenarrow beam divergence afforded by the optical link, this produces ahighly energy-efficient communications channel.

A single-aperture, multi-use telescope can be used that providesnarrow-field-of-view (NFOV) tracking and comm transmit/receive, alongwith wide-field-of-view (WFOV) link acquisition, in a single opticalassembly to reduce total system weight. This optical assembly cancomprise fast Micro-Electro-Mechanical Systems (MEMS) mirrors to provideprecision pointing and look-ahead capabilities required for the narrowoptical transmit beam.

A pointing-and-tracking beacon can be carried on the data channel via anadditional small-signal modulation. This co-use of a single laser fieldcan reduce system power and SWaP to provide precision pointing withminimal signal overhead.

A full-hemispherical mechanical gimbal can be used to provide totalhemisphere coverage. This stable and accurate gimbal can comprisearticulated joints to provide continuous field-of-regard coveragewithout the possibility for cable wrapping and tangling associated withtraditional gimbals based on rotating stages.

A controller can comprise a single FPGA. The controller can provide FECand framing of the data payload, operate the control loops for pointing-and tracking and wide-field-of-view acquisition, and communicate withthe host satellite for command and control. This single, versatilecontroller can reduce power consumption, simplify electronicsintegration, and provides an easily upgradeable solution for futureneeds.

A light-weight mechanical and opto-mechanical design can be used thatprovides not only the stability and accuracy necessary for precisionoptical alignment over a wide range of temperatures, but also the lowmass required for launch.

Individually and in combination, these novel design features can providean efficient and flexible inter-satellite communications link that canbe utilized on a wide variety of platforms, network topologies, andsatellite configurations. Additionally, use of an optical carrier, witha carrier frequency of about 3000× higher than the highest feasible RFcarrier, allows for high antenna gain in a small aperture size. Thisconfiguration can reduce both system weight and atmospheric drag inlow-earth orbit while providing inherent communications security as itis difficult to both jam and intercept transmitted optical signals.These system advantages can be coupled with a high level of pointing andtracking accuracy and precision. The size, weight, and power (SWaP)necessary to provide precise and accurate pointing is small compared tothat required for coarse pointing over the full field-of-regard, andmuch smaller than the increased SWaP that would be required to providethe same communications rate with a more divergent transmit beam due tothe necessary increase in transmit power. While wide-band signalprocessing gain could be used to reduce the required transmit power,this comes at the cost of complex signal modulation and de-modulationand increased ADC and processing requirements negating any SWaPbenefits. Precise and accurate pointing does require a significantincrease in system complexity and control, but technology advances inMEMS and FPGAs now make precision pointing possible in a low SWaPsolution.

Non-return-to-zero, on-off-keying (NRZ-OOK) can be used for signalmodulation. NRZ-OOK can provide for energy efficient modulation. NRZ-OOKallows for direct modulation of the lasers minimizing component count,and for direct digitization of the received signal using only alow-power, limiting amplifier rather than a higher-poweranalog-to-digital converter (ADC). It is the very simplicity of themodulation technique that makes it an ideal candidate for systems whichare heavily restricted in weight and power.

Pointing, Acquisition, and Tracking (PAT) can consume as much if notmore power than the communications channel. Efficient PAT can beachieved by leveraging both light-weight mechanical gimbals and MEMSmirrors to provide a pointing solution that provides a hemisphericalfield-of-regard with a pointing precision of less than about 9 μrad. Byutilizing the same FPGA for PAT, control, and communications, additionaloverhead can be kept low compared to the electronics used forcommunications alone. This low overhead, combined with a gimbal having astrong holding torque when de-powered, allows the device to completelypower down the PAT system when no link is required significantlyreducing standby power and improving total system efficiency. A singlelaser may be used for all aspects of pointing, acquisition, andtracking. This approach is novel, unique, and greatly improves terminalSWaP.

The disclosed techniques have advantages over conventional approaches.The vast majority of currently established inter-satellite links utilizeRF frequencies in the S, K, and mmW bands. However, a few testdemonstrations, notably the ESA Silex demonstration have proven thatoptical communications with precision pointing is possible forsatellite-to-satellite links. For the Silex payloads on the Spot-4 andArtemis missions, the 800 nm wavelength range was used with on-offkeying, direct detection and wavelength discrimination to isolate thetransmit and receive beams. Unlike the disclosed device, the Silexterminals had large 250 mm apertures, consumed 200 W of power, hadmasses near ˜150 kg, and provided a highly asymmetric link with datatransmitting in only one direction. While it should be noted that theseapplications were designed for Geosynchronous Equatorial Orbit (GEO) toLow-Earth Orbit (LEO) communications, the transmit laser power of 100 mWis similar to example transmit powers proposed in this disclosure. Thus,for the same laser transmit power the disclosed techniques can reducetotal system power by near 50× and weight by greater than about 100times while providing a greater field-of-regard.

Other optical-link demonstrations including the TerraSAR-X to NFIRE havedemonstrated LEO-LEO communications with data rates of 5.6 Gbps. Thesesystems however utilized a BPSK coherent communications system based onNd-YAG lasers with Tx power greater than about 1 W. These terminals hadmasses of 35 kg, and a power consumption of 120 W. However, thesedevices rely on a coherent detection system and the inefficient lasersource that causes the mass and power to be well above what is feasiblefor nano- and micro-satellites.

Compared to other small nano- and micro-satellite scale transmitters,the disclosed techniques are unique. These other devices typicallycommunicate on near 437 MHz and at rates of a few kbaud. MIT SpaceSystems Lab has designed a free-space optical module for smallsatellites, but depended entirely on body pointing so that the precisionpointing necessary for inter-satellite links would be difficult if notimpossible. Additional research has included the possibility ofincluding a fast-steering mirror for increased precision, but stillrelied on body pointing for coarse pointing of the beam. The discloseduse of a fully hemispherical steering gimbal provides a large advantageover other approaches to small inter-satellite communications and thepotential missions of their host satellites.

While other, non-mechanical approaches to beam steering have beenproposed, such as optical phased arrays, and liquid-crystal polarizationgrating switching, these have large disadvantages compared to thedisclosed approach. Optical phased arrays have poor pointing efficiencyas the greater than λ/2 spacing of optical elements reduces the fillfraction and puts a large portion of the power in secondary nodes ratherthan the central node. The liquid-crystal approach is limited by thenature of its discrete jumps as the grating is switched on and off. Incurrent products from Boulder Nonlinear Systems, these jumps occur every1.25° so that the signal would fade and be lost requiring reacquisitionevery few seconds. Furthermore, as the liquid crystal approach requiresusing polarized light, it would not allow for the use of polarizationmultiplexing to increase transmit power and provide redundancy in thetransmit laser.

FIG. 1 shows an example device 100 in accordance with the presentdisclosure. The device 100 can be a component (e.g., terminal) of asystem, such as a satellite, a network, and/or the like. The device 100can comprise components and/or devices. The device 100 can be acommunication device. The device 100 can be configured for free spaceoptical communication, such communication to and/or from a satellite.The device 100 can comprise a terminal 102 (e.g., or scope). The device100 can comprise a housing 104. The device 100 can comprise a coarsepointing device 106 configured to position the terminal 102 forcommunication. The coarse pointing device 106 may be coupled to (e.g.,attached) to the housing 104. The coarse pointing device 106 cancomprise a gimbal, gyroscope, and/or the like. The device 100 cancomprise control electronics 108. The control electronics 108 can bedisposed at least partially inside the housing 104. The controlelectronics 108 can comprise one or more circuit boards.

The device 100 can comprise a variety of subsystems, such as theterminal 102, the coarse pointing device 106, a data relay, Pointing,Acquisition, and Tracking (PAT) components, control electronics 108, andthe housing 104. The device 100 can comprise electronics, such as one ormore of the following subsystems: 1) Telescope electronics, includingthe steering mirror drivers, the quad detector front ends, and the frontend for the data detector, 2) Data relay electronics, including thelaser drivers, and interfaces to the detector front-end electronics, and3) PAT electronics, including the interfaces to the gimbal actuators,the fast-steering mirror and the point-ahead mirror, the defocusingactuator, and PAT detector. The terminal electronics can be implementedin several small boards (e.g., disposed inside the terminal 102). Thedata relay electronics, the PAT electronics, power conditioning, andcontrol can be implemented on two larger circuit boards (e.g., thecontrol electronics 108).

FIG. 2A is a block diagram showing a schematic of a transmit portion ofan example device. The transmit portion can be implemented via one ormore components disposed in the terminal 102. The transmit portion cancomprise one or more lasers 202, 204. As an example, the one or morelasers can comprise an aluminum gallium arsenide (AlGaAs) laser. A datasignal can be used to cause the one or more lasers 202, 204 to generate(e.g., modulate) one or more beams indicative of the data. The one ormore beams can be combined using a beam combiner 206, such as apolarization beam combiner. The beam combiner 206 can output a combinedbeam (e.g., single beam). The combined beam can be transmitted via anoptical interface 208 (e.g., lens, aperture) to free space.

FIG. 2B is a block diagram showing a schematic of an example receiveportion of an example device. The receive portion can be implemented viaone or more components disposed in the terminal 102. The opticalinterface 208 (e.g., aperture, lens) can receive an optical signal fromfree space. The received optical signal can be directed (e.g., via oneor more mirrors, filters) to a photodiode 210. The photodiode 210 cancomprise an avalanche photodiode, such as a silicon avalanchephotodiode. The photodiode 210 can be configured to convert the receivedoptical signal into an electrical signal (e.g., current signal). Theelectrical signal can be supplied to a first amplifier 212. The firstamplifier 212 may convert a current signal to a voltage signal, amplifythe electrical signal from the photodiode 210, and/or otherwisecondition the electrical signal. The first amplifier 212 can comprise atransimpedance amplifier (TIA). The output of the first amplifier 212can be supplied to a second amplifier 214. The second amplifier 214 canconvert the electrical signal received from the first amplifier 212 intoa digital signal. The second amplifier 214 can be a limiting amplifier.The limiting amplifier can comprise an adjustable threshold.

FIG. 3 is a graph illustrating an example optical signal transmissionwaveform. Transmit power vs. time is shown in the graph, whichillustrates a first modulation 302 and a second modulation 304 to thesame optical signal. It should be understood that the relative timescale of the two modulation speeds is not necessarily shown to scale inFIG. 3 but is only used for purposes of illustration.

The first modulation 302 can have a lower frequency than the secondmodulation 304. The first modulation 302 can comprise a beacon signal(e.g., or a dither tone). The first modulation can have a smaller depth(e.g., modulation depth, amplitude) in comparison to the secondmodulation 304. The second modulation 304 can comprise a data signal.The first modulation 302 can be used for acquisition, tracking,pointing, and/or other positioning.

The first modulation 302 and the second modulation 304 can both begenerated using the same laser source. The laser source can modulate thefirst modulation 302 and the second modulation simultaneously, as twoseparate channels, and/or the like. As an example, the first modulation302 can have a frequency in the kHz range (e.g., about 5 kHz). The firstmodulation 302 can comprise a sine wave with a frequency in the kHzrange (e.g., about 5 kHz). The second modulation 304 can have a datarate (e.g., or frequency) in the Mbps range, Gbps range, and/or thelike. The second modulation 304 can comprise an NRZ data signal (e.g.,having the data rate). The first modulation 302 can comprise a firstmodulation depth (e.g., 1 dB). The second modulation 304 can comprise asecond modulation depth different than the first modulation depth.

The first modulation 302 can be synchronously detected on aposition-sensitive, quadrant detector (e.g., spatial detector) toprovide pointing information. The pointing information can be providedto a coarse pointing element (e.g., gimbal) and a pair of fast steeringmirrors. The steering mirrors can provide precision pointing withplatform jitter compensation and look-ahead capabilities to offset thereceive and transmit directions. The steering mirrors can compriseMicro-Electro-Mechanical Systems (MEMS) fast steering mirrors canprovide a low SWaP solution configured with 1 prad of pointing accuracywith greater than 1 kHz open loop modulation response.

The device can comprise a single aperture telescope configured for NFOVpointing and tracking, WFOV acquisition, and simultaneous data transmitand receive. To provide simultaneous transmit and receive from the sameaperture wavelength multiplexing can be used. For example, while oneterminal will transmit at a first wavelength (e.g., 850 nm), theterminal can receive at a second wavelength (e.g., 830 nm). The adjacentcommunications terminal can transmit at second wavelength and receive atthe first wavelength. Wavelength division multiplexing within thetelescope, can provide transmit/receive isolation to maintain signalsensitivity. To provide both a narrow beam divergence for communicationsand NFOV tracking, and a WFOV beacon for acquisition, a lens translationstage (e.g., or defocuser) can be included so that the output field canbe defocused to provide a larger beam divergence during initialacquisition of the adjacent terminal.

FIG. 4 is a diagram showing an example terminal (e.g., scope, telescope)in accordance with the present disclosure. The terminal 400 can comprisethe terminal 102 of FIG. 1. The terminal 400 can comprise a defocuser402. The defocuser 402 can be optically coupled (e.g., via an opticalfiber) to a laser (e.g., not shown). The defocuser 402 can be configuredto receive an optical signal from the laser. The defocuser 402 can beconfigured to control a beam divergence of the optical signal. Amodulator can be configured to cause the laser to output an opticalsignal comprising a data signal and a beacon signal. The beacon signalcan be modulated at a first frequency. The data signal can be modulatedat a second frequency higher than the first frequency. The beacon signalcan be modulated at a first modulation depth (e.g., amplitude). The datasignal can be modulated at a second modulation depth greater than thefirst modulation depth. The defocuser 402 may comprise and/or becommunicatively coupled to a controller configured to cause thedefocuser 402 to adjust the beam divergence based on an operational modeof the laser.

The operational mode can comprise one or more of a first operationalmode, a second operational mode, or a third operational mode. The firstoperational mode can comprise a data communication mode. The secondoperational mode can comprise a tracking mode (e.g., or a linkacquisition mode). The third operational mode can comprise a linkacquisition mode (e.g., or a tracking mode). The controller can beconfigured to cause the defocuser 402 to adjust the beam divergence tohave a first beam divergence angle for a first operational mode. Thecontroller can be configured to adjust the beam divergence to have asecond beam divergence angle for the second operational mode. Thecontroller can be configured to adjust the beam divergence to have athird beam divergence angle for the third operational mode. The secondbeam divergence angle can be larger than the first beam divergenceangle. The third beam divergence angle can be larger than the first beamdivergence angle and/or second beam divergence angle.

The terminal 400 can comprise one or more steering mirrors 404 (e.g.,fast steering mirrors). The one or more steering mirrors 404 can beconfigured for adjusting pointing (e.g., of a signal for transmission ora received signal) during one or more of a tracking mode or a datacommunication mode. The defocuser 402 can supply an optical signal toone of the steering mirrors 404. The steering mirror 404 can supply theoptical signal to a first filter 406 (e.g., transmission receivefilter). The first filter 406 can supply the optical signal to a secondone of the one or more steering mirrors 404. The second one of the oneor more steering mirrors 404 can supply the optical signal to a secondfilter 408. The second filter 408 can comprise a stray light filter.

The second filter 408 can supply the optical signal to an opticalinterface 410. The optical interface 410 can be configured to receivethe optical signal from the defocuser 402 (e.g., or via the secondfilter 408). The optical interface 410 can be configured to output theoptical signal into free space. The optical interface 410 can compriseone or more openings, one or more lenses, and/or the like. The opticalinterface 410 can comprise a first aperture (e.g., as shown in FIG. 1and FIG. 4.). As an example, the size of the first aperture can comprise46 mm. The optical signal can be output via the first aperture duringone or more of the first operational mode, the second operational mode,or the third operational model. The optical interface can comprise asecond aperture (e.g., as shown in FIG. 6). The optical signal can beoutput via the first aperture during a first operational mode. Theoptical signal can be output via the second aperture during the secondoperational mode and/or the third operational mode.

The optical interface 410 can be configured to receive an optical signalfrom free space. The optical interface 410 can be configured to supplythe received optical signal to the second filter 408. The second filter408 can be configured to supply the received optical signal to thesecond mirror of the one or more steering mirrors 404. The second mirrorof the one or more steering mirrors 404 can be configured to supply thereceived optical signal to the first filter 406. The first filter 406can be configured to reflect the received optical signal to a thirdfilter 412. The third filter 412 can be configured to supply thereceived optical signal to a beam splitter 414.

The beam splitter 414 can be configured to split the received opticalsignal into a first signal and a second signal. The beam splitter 414can be configured to supply the first signal to a data detector 416. Thedata detector 416 can be optically coupled to the beam splitter 414(e.g., directionally oriented, aligned for optical transmission). Thedata detector 416 can be configured to receive the first signal. Thedata detector 416 can be configured to convert the first signal into adata signal (e.g., electrical data signal, digital data signal. The datadetector 416 can comprise a photodiode configured to convert, based onthe data signal, the first signal to an electrical signal.

The beam splitter 414 can be configured to supply the second signal to aspatial detector 418. The spatial detector 418 can be optically coupledto the beam splitter 414. The spatial detector can be configured toreceive the second signal. The spatial detector can be configured toconvert, based on the beacon signal, the second signal to an electricalsignal for determining positioning information. The positioninginformation can be used to adjust one or more of a coarse pointingelement (e.g., a mechanical gimbal) or the one or more steering mirrors404.

One or more controllers can be configured to control operation of theelements of the terminal 400, such as the defocuser 402, the one or moresteering mirrors 404, and/or the like. The defocuser 402 and the one ormore steering mirrors 404 can be controlled by separate controllers. Amaster controller can control the separate controllers. The mastercontroller can comprise a single FPGA (e.g., or other integratedcircuit, such as an ASIC).

The master controller can be configured to control multiple modes ofoperation, such as a mode to establish the link, a mode to maintain thelink, and a mode to close the link. The one or more controllers (e.g.,or master controller) can control operation of communications terminalsas follows. Command and control from the host satellite can signal acommunications request and/or provides ephemeris data for the desiredsatellite node. One or more lasers can be turned on (e.g., at fullpower). The output (e.g., combined output) of the one or more lasers canbe defocused to provide a wide-field-of-view beacon for acquisition. Theone or more lasers can be modulated with a beacon tone (e.g., atsubstantially 100% with a ˜5 kHz acquisition tone). Detector boards, agimbal, and MEMS control boards can be powered on.

A transmitted beam with a large beam divergence can be raster scannedover the expected satellite location determined from ephemeris data. Thetwo ends of the link can scan at different speeds to ensure each devicewill point at each other and establish lock in a short time (e.g., lessthan about 72 s). Locking can be signaled by both satellites making asmall change in their beacon's respective modulation frequency. Atransmit laser output can be focused (e.g., by defocuser 402) to providea narrow diverging beam for communications and precise pointing andtracking. Both terminals can send handshake data to establish timing andthe communications rate. The beacon modulation can be reduced to 1 dB,as shown in FIG. 3.

Payload data from the host satellites can be transmitted. Ephemerisdata, pointing calibration data taken during acquisition, and/orpositive feedback from the spatial detector(s) 418 (e.g., one or morequad photodetectors), can provide control signals to the coarse pointingelement and/or one or more steering mirrors (e.g., MEMS mirrors) forcontinued precision pointing over the link's lifetime. Command andcontrol from the host satellite can request the termination of thecommunications link. Transponder and pointing and tracking operationscan be de-powered. The one or more controllers (e.g., the FPGA, themaster controller) can enter a standby state to reduce power consumptionuntil a communications link is again requested.

FIG. 5 is a diagram showing another example terminal 500 in accordancewith the present disclosure. The terminal 500 can comprise some or allof the features of the terminal 400 of FIG. 4. The terminal 500 beconfigured for only using one laser and/or one aperture. Only a singlespatial detector 418 can be used for the first operational mode, thesecond operational mode, and/or the third operational mode. The terminal500 can comprise a zoom element 502. The zoom element 502 can be used toincrease the Field of View of the terminal 502 during Acquisition,rather than having a completely separate (e.g., and fixed) Wide Field ofView Aperture and associated additional optical elements.

The zoom element 502 can be optically coupled between the spatialdetector 418 and the beam splitter 414. The zoom element can beconfigured to control one or more of a beam divergence, a focus, a fieldof view, and/or the like of the received optical signal (e.g., or thesecond signal from the splitter 414) upon the spatial detector 418. Oneor more controllers can be configured to cause the zoom element 502 toadjust the beam divergence (e.g., or field of view, configuration)between a first beam divergence (e.g., or a first field of view, firstconfiguration) for a first operational mode and a second beam divergence(e.g., second field of view, second configuration) for a secondoperational mode. The second beam divergence can be larger (e.g., wider)than the first beam divergence. Additionally or alternatively, the firstbeam divergence can be wider than the second beam divergence.

The zoom element 502 can comprise a first configuration associated withthe first operational mode. The zoom element 502 can comprise a secondconfiguration associated with the second operational mode. The zoomelement 502 can comprise a third configuration associated with thesecond operational mode. The first configuration can focus (e.g., ordefocus) an optical signal having a first beam divergence upon thespatial detector (e.g., with an appropriate focus and/or beam divergencefor the spatial detector). The second configuration can focus (e.g., ordefocus) an optical signal having a second beam divergence upon thespatial detector (e.g., with an appropriate focus and/or beam divergencefor the spatial detector). The first configuration can focus (e.g., ordefocus) an optical signal having a first beam divergence upon thespatial detector (e.g., with an appropriate focus and/or beam divergencefor the spatial detector).

FIG. 6 shows an example terminal 600 comprising multiple apertures. Theterminal 600 can comprise a first aperture 602 and a second aperture604. The first aperture 602 can be larger than the second aperture 604.The first aperture 602 can be used for a first operational mode, asecond operational mode, a third operational mode, and/or the like. Thesecond aperture 604 can be used for the second operational mode and/orthe third operational mode. The first aperture 602 can be used fortransmitting the laser signal. The first aperture 602 can be used foracquisition, tracking, data transmission, or a combination thereof. Thefirst aperture 602 can be used for narrow field of view tracking. Thesecond aperture 604 can be used for wide field of view acquisition.

FIG. 7 shows a diagram of example optical elements for the secondaperture of FIG. 6. The optical elements can be enclosed in the terminal600. The optical elements can comprise an additional spatial detector702. The terminal 600 can comprise both the spatial detector 418described above and the additional spatial detector 702. The opticalelements can be configured for supplying a wide field of view signal tothe additional spatial detector 702. The optical elements can comprise afilter 704. The filter 704 can comprise an apodization filter.

FIG. 8 is flowchart showing an example method 800 for free space opticalcommunication. At step 802, a first optical signal can be generated. Thefirst optical signal can be generated by one or more lasers. The firstoptical signal can comprise a first data signal and a first beaconsignal. The first beacon signal can be modulated at a first frequency,and the first data signal is modulated at a second frequency higher thanthe first frequency.

At step 804, a beam divergence (e.g., focus, or other opticalcharacteristic) of the first optical signal can be adjusted. Thedefocuser 402 of FIG. 4 can adjust the beam divergence. The beamdivergence can be adjusted based on an operational mode of the laser.Adjusting the beam divergence of the first optical signal can comprisecausing a defocuser to adjust the beam divergence to have one or more ofa first beam divergence for a first operational mode or a second beamdivergence for a second operational mode. The second beam divergence canbe wider than the first beam divergence. The first operational mode cancomprise a data communication mode. The second operational mode compriseone or more of a tracking mode or a link acquisition mode.

At step 806, the first optical signal can be output in to free space.The first optical signal can be output via an optical interface (e.g.,via a first aperture, a lens). The first optical signal can transmitinformation from one device to another (e.g., from satellite tosatellite, from ground to satellite, from satellite to ground, from aspacecraft to another spacecraft, from a spacecraft to a satellite, froma spacecraft to ground).

At step 808, a second optical signal can be received. The second opticalsignal can be received via the optical interface (e.g., via the firstaperture, via a second aperture). The second optical signal can comprisea second data signal and a second beacon signal. The second beaconsignal can be modulated at a first frequency. The second data signal canbe modulated at a second frequency higher than the first frequency.

At step 810, the second optical signal can be split (e.g., by a beamsplitter) to a third optical signal and a fourth optical signal. Thesecond optical signal can be split by the beam splitter 414 of FIG. 4.

At step 812, positioning information can be output. The positioninginformation can comprise a positioning signal. The positioninginformation can be output by a spatial detector, such as a quaddetector. The spatial detector can detect the third optical signal. Thepositioning information can be based on which portions of the spatialdetector detect the third optical signal. The positioning informationcan be output based on the second beacon signal. The second beaconsignal can indicate which device is transmitting the second opticalsignal.

A coarse pointing element can be adjusted based on the positioninginformation. The coarse pointing element can be configured for adjustingpointing during an acquisition mode (e.g., link acquisition mode). Oneor more steering mirrors can be adjusted based on the positioninginformation. The one or more steering mirrors can be configured foradjusting pointing during one or more of a tracking mode or a datacommunication mode.

At step 814, communication information can be output. The communicationinformation can comprise data, a digital signal, an electrical signal,and/or the like. The communication information can be output based onthe fourth optical signal. The fourth optical signal can be received bya data detector, such as an avalanche photodiode that converts theoptical signal into an electrical signal. The electrical signal can beamplified and/or converted into a digital signal. The communicationinformation can be based on the second data signal. The data, electricalsignal, digital signal and/or the like can be indicative of the datacarried in the second data signal. Outputting the communicationinformation can comprise generating, using the photodiode, a currentsignal indicative of the data signal, converting the current signal to avoltage signal, and converting the voltage signal to a digital signal.

Examples and Analysis

The following description provides further examples and analysis. Itshould be understood that the disclosure is not limited to theseexamples, but the examples are provided for purposes of illustration.

FIG. 9 is a block diagram showing an overall system architecture of anexample device. Both the data and the signaling beacon can be generatedfrom the same 850/830 nm source. To achieve sufficient Tx/Rx isolationin the common aperture, the optical modules can transmit and receive atdifferent wavelengths. For initial acquisition of the link, the transmitsignal can be defocused to increase the beam divergence and decrease thelink acquisition time. During data transmission, the beam can be focusedto increase the received power and data rate. The received signal can bebroken into two optical paths using an optical beam splitter; one pathcan provide data reception using a high-speed APD, while the second pathcan provide precision pointing and tracking information via aposition-sensitive, quadrant detector.

For a single transmit laser to provide both data and signaling, twoseparate modulation schemes can be used simultaneously. Data can betransmitted via a high-speed, NRZ-OOK modulation, while the beacon willbe provided by an additional slow, small-signal modulation. PATinformation can be acquired through synchronous detection of thissmall-signal modulation. Payload data acquisition can utilize a limitingamplifier to digitize the high-speed data without the need for an ADCreducing the total power draw of the electronics. This approach providesrobust pointing and tracking, and high-speed data acquisition in asingle aperture telescope greatly reducing system weight and power tomeet the program goals.

A single FPGA can provide full system control including communication tothe host satellite, PAT control, data framing, FEC, Tx, and Rx.

Detailed subsystem descriptions and break-downs are provided in thefollowing sections.

The optical source (e.g., laser) can be directly modulated. The opticalsource can comprise two polarization multiplexed and directly modulatedFabry-Perot laser diodes. The laser diodes can have of wavelength ofeither 830 nm or 850 nm. Polarization multiplexing provides systemredundancy in that if a single laser fails the system can continue tooperate with near full functionality. The output light can be on-off keymodulated, with an additional small-signal modulation for PAT via directmodulation. During the acquisition phase when no data are present, thebeacon will utilize a large-signal modulation to improve detection SNR.

The photodetector can comprise silicon avalanche photodiode (APD). Thephotodetector can be temperature stabilized to 300 K using a TEC. Thesignal from the APD can be first amplified in a transimpedance amplifierwith gain control. The amplified signal can be passed through a limitingamplifier with adjustable threshold to make the I/O bit decision (e.g.,removing the need for an ADC). Additionally, the receiver bandwidth canbe configured below the data rate to optimize BER based on receivernoise and inter-symbol interference.

Pointing, Acquisition, and Tracking (PAT) is described as follows.Pointing, Acquisition, and Tracking (PAT) are important functions toclosing the data link. The PAT system at a high level can have twooperating modes: the acquisition mode (e.g., where the two ends of thelinks scan to make a connection) and the comm mode (e.g., datacommunication mode, where the systems maintain the link with minimalpower fluctuation). The PAT system can comprise a gimbal that providescoarse beam steering over the required field of regard. The PAT systemcan comprise a fast steering mirror that provides fine pointing accuracyand fast pointing changes. The PAT system can comprise a second faststeering mirror that offsets the pointing of the transmit beam relativeto the receive direction to compensate for the relative motion of thetwo satellites and also to rapidly scan the transmit beam in acquisitionmode. The PAT system can comprise a spatial detector, such as a quaddetector (e.g., quad PIN Si detector), configured to sense the pointingdirection that feeds back to the fast steering mirror and gimbal. Thespatial detector can have a total field of view of 2 mrad. A singletransmit beam can be used to provide both data communication andpointing tracking. The signal delivered to the spatial detector can bederived from a 20% tap off the signal delivered to the APD.

To initially acquire the adjacent satellite, a wide-field-of-view (WFOV)acquisition mode can be used to establish the correct pointing vector.This can be done using the same NFOV optics and emulating a WFOV beamthrough fast scanning of the MEMs mirrors. In addition, the lasercollimating lens can be translated to increase the diffraction limiteddivergence to 67 μrad and the laser can be modulated at −5 KHz. In theacquisition mode, data communication can be disabled. In the acquisitionmode the full laser power can be utilized for PAT (e.g., increasing theaverage received power by 4 dB). The acquisition method describedassumes that there is no communication between the satellites; with somecommunication, faster acquisition schemes could be enabled.

The relative motion of the two satellites together with the small beamdivergence at IR wavelengths suggests that the pointing vector of thetransmit beam should be offset from the pointing vector of the receivebeam. The transmitter can look ahead of the receiver. This pointingdifference can be on the order of 900 μrad at the worst case at 4000 km.It can be assumed that the expanded beam angle of the WFOV is largerthan the uncertainty in the look-ahead provided from ephemeris data. Thelook-ahead angle can be optimized by imposing a small nutation on thetransmit beam which can be detected on the opposite side of the link.This pointing-error data can be relayed between the two satellites byvarying the modulation frequency of the beacon synchronously with thenutation which can be processed to optimize the pointing offset. Thisoptimization can be implemented just before the WFOV beam is narrowedfor communications. During data communications the supervisory channelcan provide the necessary feedback to allow closed-loop operation of thelook-ahead mirrors.

When commanded to establish a link, first the ephemeris data can beanalyzed to provide coarse pointing. As an initial estimate, it can beassumed that there is a high probability of finding the target within afield of regard (FOR) equal to a +/−3 mrad cone around the pointingdirection given by the ephemeris data. Next the laser steering mirrorcan be continuously scanned over the quad detector field of view (QFOV)which is +/−1 mrad and offset by the ‘look ahead’ angle. The time toperform a single scan can be 0.44 s and can be set by the time the spotmust be on target to achieve a threshold SNR level. Simultaneously, theQFOV can be scanned slowly over the FOR. In the simplest implementation,the two satellites would perform spiral scans over the FOR at differentrates, the ratio of the rate being equal to that of the QFOV solid angleto FOR solid angle. The two scan times with this implementation are 5.6s and 72 s, with some margin added for overlap within a scan. The worstcase acquisition time can be the 72 s time. More complex scanningalgorithms can be envisioned to improve the locking time. Digitalsignaling can be provided between the satellites by shifting themodulation frequency. A transmitter can shift its modulation frequencywhen the transmitter detects and locks to the received beacon. The linkcan transition to the operating mode when it is both receiving andtransmitting the shifted frequency. The transition can comprise acontrolled reduction in the laser scan range followed by the addition ofdata modulation. This can be followed by bringing up the data link,timing, framing, FEC, etc. During standby no power can be required andthe devices are completely deactivated. Weight contributions from thesecomponents can be included in the electronic PCBs and telescope.

An example telescope (e.g., or terminal) is described as follows. Thetelescope can comprise a light-weight, multifunction telescope toperform satellite acquisition, tracking and data communications. Thistelescope architecture can be seen in FIG. 4, and a full opticalsimulation of the system is shown in FIG. 10 showing that the fulloperation can be supported within a compact optical payload. In FIG. 10,the receive path is shown in dark lines, transmit path in white lines.The telescope can comprise an opto-mechanical design shown in FIG. 11.The telescope can have a compact refracting configuration which has aprimary aspheric lens of 46 mm diameter and a focal length of 40 mm. Thesecondary lens can have a focal length of 1.8 mm giving the telescope amagnification of 22 x and collimated beam diameter of 2 mm within theoptics system. This high magnification and narrow collimated beam sizeallow for utilizing light-weight, high-speed MEMS mirrors for bothfast-steering and look-ahead pointing, reducing the total telescopeweight.

The telescope can perform simultaneous transmit and receive bywavelength multiplexing transmit and receive beams at 850 and 830 nm.During communications, the laser source can be launched from an opticalfiber and collimated into a diffraction limited beam. The diffractionlimited beam can have a beam divergence of 15 μrad (e.g., with anexpected actual beam divergence of 27 μrad due to non-ideal optics). Afirst MEMS mirror can provide look-ahead capabilities to account for theoffset between the transmit and receive directions required by therelative motion of the satellites and the time of flight of light. Asecond MEMS mirror can be configured as a fast-steering mirror to removehigh-speed jitter due to platform vibrations and provide the requiredprecision pointing. During initial acquisition of the adjacentsatellite, a lens translator can be used to defocus the transmittedlaser light providing a wider transmit beam divergence and reducing thetime to acquisition.

After the primary telescope, a band-pass filter can be used to reducebackground star and planet light that enters the optical path. Thereceive path can be separated from the transmit path using a dichroicfilter. A second transmit/receive filter can be used to provideadditional isolation between the received signal and any back reflectedtransmit light. The received beam can be split with a beam splitter. 20%of the received signal can be directed onto a quad position sensingdetector for alignment to the adjacent satellite. 80% of the receivedbeam can be focused onto a high-speed Si APD for data reception.

One key challenge of the telescope's opto-mechanical design is mountingthe optics rigidly enough that they maintain alignment during launch,and over the mission lifetime while keeping the total telescope weightlow. All parts in the telescope can be hard mounted to the opticalplatform and designed to withstand launch conditions. Lens and opticalmounts can be designed to allow adjustment for ease of assembly and tosecurely hold that adjustment during launch and deployment.Considerations can be made to provide a thermally and mechanicallystable optical platform by specifying controlled thermal expansionmaterials where necessary, while using lightweight parts andconstruction techniques to stay within the weight budget. The lenstranslation stage and MEMS mirrors can provide multiple methods ofre-aligning the optical train after launch if necessary. MEMS steeringmirrors can be chosen for their low mass compared to otherelectromagnetic and piezoelectric solutions with similar pointingaccuracies and control bandwidths.

An example coarse pointing device, such as a gimbal is described asfollows. FIG. 12 shows a mechanical model of a gimbal in the stowedposition. The gimbal can comprise the COBRA gimbal produced by TethersUnlimited. The gimbal can comprise continuous, full-hemisphericalcoverage with a pointing accuracy of better than 300 μrad. The gimbalcan have a mass of only 170 g including actuators and fits in a standardcubesat footprint of 10×10 cm². The gimbal can have a large holdingtorque such that it is rigidly held in place with no power, and a centerthrough hole to allow cables to be routed to the payload withoutcatching over the total field-of-view. Tethers can be used to customizethe gimbal to reduce the overall power draw from 1.5 to 0.5 W. This canbe done by reducing the maximum slew rate from 120°/s to 6°/s reducingthe power required for the drive motors. During launch, a burn wire canconstrain the gimbal in the axial direction (perpendicular to themotors) and the base and distal ends of the gimbal would have a cup-coneinterface for lateral support. Deployment can be initiated by anon-explosive actuator (NEA). The gimbal's actuators can provide lowmechanical noise performance due to their ability to microstep andcoupling to a planetary gearhead. Thus, minimal jitter should beimparted to the host satellite. However, if lower noise is necessary forlong-exposure images, the mechanical gimbal can be completely disabled,and PAT can be performed exclusively using the fast-steering mirror forseveral seconds.

While the COBRA gimbal provides a mature baseline solution, the per unitcost may make it prohibitively expensive for a low cost solutiondeployable on a wide range of platforms. For this reason, low-power, 3-Dstabilized quadcopter gimbals, can be used. These gimbals are widelyavailable with prices <$200 per unit and provide the specified slew andpointing accuracy in a light weight form factor, but are not designedfor space environments. We will procure a sample, examine its spaceworthiness, and engineer mechanical and electrical customizations tooptimize it for space flight, with a focus on reducing their total powerdraw which is typically several Watts.

Example processing, Controls, and Power Electronics are described asfollows. The electronics can be divided into two physical enclosures—thetelescope and the base. The structure of the electrical subassemblies(PCBs) can be driven by the configuration of the optical systems, themechanical design, and the optimization of electrical interconnects forsignal integrity. The control electronics along with power conditioningand drive circuitry can be housed in the base unit on two boardsreferred to as the main controller and the power/driver boards. The maincontrol board can comprise an FPGA and/or support devices, such asclocks, memory, and power. The firmware can reside on the main controlboard in the FPGA and can be responsible all control functions andcommunications. The power/driver board can provide system powerconditioning, source lasers and their associated drive electronics,and/or the gimbal drive electronics. The remaining PCBs can be disposedin the telescope (e.g., terminal) and provide drive circuitry for MEMSand electro-optic devices that reside there. The main control board canmonitor and/or drive the telescope electronics through a wire harnessconnecting the base and telescope. There can be one or more (e.g., four)small active circuit boards in the telescope. The telescope boardsperform optical to electrical conversion at the photo diodes along withdriving the MEMS steering mirrors and the FOV actuator.

Example FPGA platforms that can be used are listed in Table 1, withflight heritage or a reasonable path to flight. The FPGA will serve asthe singular control device for all electro-optic and electricalsystems. A highly integrated FPGA technology can be used along withavailable commercial IP, such as forward error correction (FEC). Thisstrategy also provides a simpler path to an ASIC to further improvepower and weight contributions for larger scale deployments incommercial and government markets.

TABLE 1 Vendor Family Part Xilinx Zynq XC7Z020 Altera Aria V 5AGTFC7HXilinx Spartan XC6SLX75 Microsemi SmartFusion2 M2S050T

The PAT electronics can be distributed throughout the electronics acrossmultiple circuit boards with firmware residing on the main controlboard. The PAT electronics and software can be configured to controlboth coarse and fine pointing. Two interacting control loops withmultiple control points can be used for the coarse and/or fine pointing.The FPGA can generate waveforms for modulation over the transmit lasersfor each control loop. The DSP within the FPGA will read the quaddetector ADC and perform numerical conversion, scaling, and filtering ofthe received signal. The internal algorithm can interpret this inputalong with ephemeris data from the host satellite and drive the pointinghardware to converge on a lock with the target satellite. The firmwarecan transition from a standby phase, to an acquisition phase, and thento the data transfer phase. The acquisition phase (e.g., operationalmode) can comprise a coarse pointing phase that uses the gimbal andsteering mirrors to align to the transmitted beacon in the wide FOV.Once acquisition is complete, the fine steering mirror can be furtherused to track the narrow FOV beacon. The electronics can be configuredfor maintaining these phases and performing power reduction by turningoff and/or limiting power to systems not in use during each phase.Additional techniques will be employed within the FPGA to further reducepower such as clock gating.

The data path for the system can flow between the FPGA and thepayload-data bus and between the FPGA and the transmit/receive optics.Optical Transport Network (OTN) protocol can be used. OTN provides aframing structure and FEC option along with payload allocation thatsupports Ethernet along with standard telecom protocols as well. The OTNprotocol supports general FEC (GFEC) that imposes a 7% overhead. The OTNsolution is available from FPGA vendors and third parties as a logiccore. The payload interface may not defined in the BAA. OTN's OPU can beused. However, the selection of the physical layer and upper layerprotocols can have an impact on power consumption. In addition to thesefunctions the data path processor can be configured to negotiate thehighest possible link rate based on received signal strength. The systemcan also provide telemetry such as link rate, statistics, and FEC dataas power budgets allow. While OTN rates are significantly higher thanthose that will be used, the framing, FEC, overhead and other functionsdefined in OTN can be utilized by running the data clock at anappropriate rate.

Link negotiation can begin at the lowest supported rate and increase toa rate that exceeds acceptable threshold and back-off to previous rate.This threshold can be determined in cooperation with the customer systemengineer once the contract has been executed. The pre-FEC rates used forthe purpose of this proposal can comprise 70 Mbps, 225 Mbps, 450 Mbps,and 640 Mbps. The number of rate selection can be limited to four toreduce complexity; however, the rates themselves can be adjusted to fita particular network architecture and satellite node configuration.

The control plane can comprise a processor, an interface to theplatform, and/or all of the measurement points available in theelectronics. The electronics will provide telemetry to the platform thatincludes customer specified measurements and statistics. The controlplane will also allow the platform to provide ephemeris and IMU data tothe electronics. The firmware can be updated through the payloadinterface allowing for field enhancements.

Example Packaging and Assembly are described as follows. The discloseddevice can comprise two major subassemblies: 1) the main electronicsenclosure assembly which provides the housing for the main controllerand power boards of the system and also provides the mounting andsupport for the motorized Cobra™ Gimbal, and 2) the optical telescopewhich houses the optical lenses, MEMS mirrors, filters and relatedcomponents for pointing acquisition and tracking control. Connecting thetwo subassemblies can be the motorized gimbal. Aluminum alloy withchemical conversion coating as per MIL-DTL-5541 can be used to constructthe main electronics enclosure. Material will be machined out from thebase and top cover to reduce overall weight of enclosure assembly. Thepreliminary enclosure concept is shown in FIG. 13.

The base housing can be machined from a single piece of aluminum toprovide maximum mechanical strength. The base housing can includemounting feet for secure fastening to the host satellite. It can havefour bosses to provide mounting for the PCB assemblies. Appropriatestandoffs height will be used to stack up multiple PCB assemblies whichwill be secured using machine screws. Thermal interface pads can beplaced under the high heat dissipating components of PCB to sink heat tothe metal body of enclosure. The top plate can provide mounting holesfor the motorized Cobra™ gimbal assembly. The power and I/O signalconnectors can be placed on one side of the enclosure to bring the powerin from the host satellite. The enclosure can be enveloped inspace-qualified multi-layer insulation (MLI) to provide temperaturestabilization. The MLI can provide protection from micro-meteorites anda common grounding plane to prevent arcing. MLI can be sufficientprotection from micro-meteorites and other debris. Based on dataprovided by the ESA's post-flight analysis of Eureca only two impactevents of significance are expected, neither of which is likely tocompletely penetrate the MLI.

SWaP Related Issues. The disclosed techniques can be used to implement adevice that would be a very compact, low weight, low-power consumingoptical communications terminal, capable of bi-directional high-speedcommunications. Such a device would have obvious benefits forspaced-based communications networks, where payload size, mass and powerefficiency are of prime importance.

The device may be used to implement a space-based communication network.The communication network may comprise a plurality of space objects,such as satellites, space stations, space ships, and/or the like. Thecommunication network can comprise a satellite constellation network.The communication network can comprise a mesh network. The communicationnetwork can be configured to provide a global network for connecting tothe internet via communications between the ground and space objects. Incomparison to conventional devices, using the disclosed techniquesallows for a much smaller, cheaper terminals. Furthermore, for true meshnetwork inter-connectivity, the space objects may be required to have atleast 4 or 5 communications terminals on a single satellite, making theSWaP per terminal an even bigger concern.

Weight issues. An example optical communications terminal can comprise(e.g., or consist of) three parts: the optical telescope, the coarsepointing gimbal, and the control electronics. The disclosed techniquescan be used to improving the implementation of a small, compact,efficient telescope. If one has a small compact efficient telescope, itbecomes possible to combine this with a small, compact, efficientGimbal, to achieve all the desired functionality required of an opticalcommunications terminal. Use of such a small, compact, efficient Gimbalwould not work for larger telescopes.

Using the disclosed techniques (e.g., which may be implemented in COTShardware) it is possible to build a fully functioning telescope thatweighs entirely less than about 1 pound. A compact, coarse pointing COTSgimbal may be used to meet pointing requirements, when using a telescopethat weighs less than 1 lb. Such gimbal may weigh less than about 0.4lbs. The base electronics which controls all aspects of LinkPerformance, acquisition, Tracking and Data Link creation, maintenancecan be implemented with electronics boards that weigh less than about0.75 lb. Thus, a fully functioning optical terminal can be made that canprovide the necessary function for implementing high-speed lasercommunications, and still weigh entirely less than about 2.5 pounds.

Power Consumption Issues. The on-orbit electrical power consumption isanother important performance metric that distinguishes the disclosedtechniques from convention devices. That is, the ability to perform allnecessary link Acquisition, Tracking and Data Link functions withminimal electrical power consumption.

With any optical communications link, there can be different modes ofoperation: Standby Mode: No Acquisition or Data Transfer occurring,minimum power consumed required for “life support”; Acquired Mode:Terminal is actively searching for its companion terminal, notably theGimbal is being fully exercised in this search mode; Data Transfer Mode:Link has been Acquired and data transfer has begun. The orbit-averageelectrical power consumption for an example COTS implemented opticalcommunications terminal can be expected to be less than about 3.5 watts.

FIGS. 14A-B show an example defocuser 1400 adjusted for different beamdivergences. The defocuser 1400 can be configured to receive an opticalsignal. The defocuser 1400 can comprise the defocuser 402 of FIG. 4. Theoptical signal can be received from a laser. The laser can transmit theoptical signal to the defocuser 1400 via a fiber 1402. The fiber 1402can output the optical signal as a free space optical signal. Thedefocuser 1400 can be configured to control a beam divergence of theoptical signal. The optical signal can comprise a data signal, a beaconsignal, or a combination thereof. A controller can be configured tocause the defocuser 1400 to adjust the beam divergence based on anoperational mode of the laser. The defocuser 1400 can enable use of thelaser and output optics for both a communication mode and one or more ofa tracking mode or a link acquisition mode. The output optics cancomprise the one or more steering mirrors 404, the first filter 406, thesecond filter 408, the optical interface 410, or a combination thereof.

The defocuser 1400 can comprise an adjustable lens 1404 configured tocontrol the beam divergence. The defocuser 1400 can be configured tocontrol the beam divergence by adjusting a distance between theadjustable lens and a source of the optical signal (e.g., or anotherlens, such as a lens at the output of the fiber 402, or a lens of thebeam expander 1410). The controller can be configured to cause thedefocuser 1400 to adjust the beam divergence to a first beam divergence1406. FIG. 14A shows an example defocuser 1400 adjusted for a first beamdivergence. The first beam divergence can result in a substantiallycollimated beam. The first beam divergence 1406 can be for (e.g., orassociated with) a first operational mode. The controller can beconfigured to adjust the beam divergence to have a second beamdivergence. The second beam divergence can be for (e.g., or associatedwith) a second operational mode. FIG. 14B shows an example defocuser1400 adjusted for a second beam divergence. The second beam divergencecan be larger (e.g., wider) than the first beam divergence. The firstoperational mode can comprise a data communication mode. The secondoperational mode comprise one or more of a tracking mode or a linkacquisition mode.

The defocuser 1400 can modify an optical signal to have the first beamdivergence by adjusting the defocuser 1400 to the first setting. Thefirst setting can comprise a position, location, shape, combination,and/or the like of one or more lenses. Adjusting the defocuser 1400 tothe first setting can comprise adjusting a distance between theadjustable lens 1404 and a source of the optical signal to a firstdistance (e.g., Δ×1).

The defocuser 1400 can modify an optical signal to have the second beamdivergence by adjusting the defocuser 1400 to the second setting. Thesecond setting can comprise a position, location, shape, combination,and/or the like of one or more lenses. Adjusting the defocuser 1400 tothe second setting can comprise adjusting a distance between theadjustable lens 1404 and a source of the optical signal to a seconddistance (e.g., Δ×2).

The defocuser 1400 can comprise a beam expander 1410. The beam expandercan be configured to increase the size of a beam comprising the opticalsignal. The beam expander 1410 can be configured to receive the opticalsignal from the adjustable lens 1404. The beam expander 1410 cancomprise one or more lenses configured to increase a size of the opticalsignal. The beam expander 1410 can be configured to output the opticalsignal with the first beam divergence and/or the second beam divergence(e.g., and any other beam divergence)

FIG. 15 is a graph showing beam divergence and lens spacing for anexample defocuser. The graph shows that small changes in lens spacingcan cause sufficient changes in beam divergence to enable multipleoperational modes for communication, tracking, and link acquisition.

FIG. 16A shows another example defocuser 1400. The adjustable lens 1404of the defocuser 1400 can comprise a liquid lens configured to changeshape to modify the beam divergence. The shape (e.g., and beamdivergence) can be changed in response to an electrical signal (e.g.,applied to the liquid lens), such as a voltage signal. Changing theshape can comprise changing a radius of curvature of the liquid lens.FIG. 16B shows a graph illustrating how the lens radius of curvature canbe varied based on different voltages to achieve different beamdivergence results.

The defocuser described herein can be used to enable a system for freespace optical communication. The system can comprise a first opticalterminal. The system can comprise a second optical terminal. The secondoptical terminal can be configured to communicate with the first opticalterminal. One or more of the first optical terminal or the secondoptical terminal can comprise the defocuser as described herein. Thefirst optical terminal can be disposed on a space object (e.g.,satellite, space shuttle, space station, moon or planet base). Thesecond optical terminal can be disposed on a second space object. Thefirst space object and the second space object can be part of a meshnetwork, constellation network, low earth orbit network, a combinationthereof, and/or the like.

FIG. 17 depicts a computing device that may be used in various aspects,such as the one or more controllers described herein may each beimplemented in an instance of a computing device 1700 of FIG. 17. Thecomputer architecture shown in FIG. 17 shows a conventional servercomputer, workstation, desktop computer, laptop, tablet, networkappliance, PDA, e-reader, digital cellular phone, or other computingnode, and may be utilized to execute any aspects of the computersdescribed herein, such as to implement the methods described herein.

The computing device 1700 may include a baseboard, or “motherboard,”which is a printed circuit board to which a multitude of components ordevices may be connected by way of a system bus or other electricalcommunication paths. One or more central processing units (CPUs) 1704may operate in conjunction with a chipset 1706. The CPU(s) 1704 may bestandard programmable processors that perform arithmetic and logicaloperations necessary for the operation of the computing device 1700.

The CPU(s) 1704 may perform the necessary operations by transitioningfrom one discrete physical state to the next through the manipulation ofswitching elements that differentiate between and change these states.Switching elements may generally include electronic circuits thatmaintain one of two binary states, such as flip-flops, and electroniccircuits that provide an output state based on the logical combinationof the states of one or more other switching elements, such as logicgates. These basic switching elements may be combined to create morecomplex logic circuits including registers, adders-subtractors,arithmetic logic units, floating-point units, and the like.

The CPU(s) 1704 may be augmented with or replaced by other processingunits, such as GPU(s) 1705. The GPU(s) 1705 may comprise processingunits specialized for but not necessarily limited to highly parallelcomputations, such as graphics and other visualization-relatedprocessing.

A chipset 1706 may provide an interface between the CPU(s) 1704 and theremainder of the components and devices on the baseboard. The chipset1706 may provide an interface to a random access memory (RAM) 1708 usedas the main memory in the computing device 1700. The chipset 1706 mayfurther provide an interface to a computer-readable storage medium, suchas a read-only memory (ROM) 1720 or non-volatile RAM (NVRAM) (notshown), for storing basic routines that may help to start up thecomputing device 1700 and to transfer information between the variouscomponents and devices. ROM 1720 or NVRAM may also store other softwarecomponents necessary for the operation of the computing device 1700 inaccordance with the aspects described herein.

The computing device 1700 may operate in a networked environment usinglogical connections to remote computing nodes and computer systemsthrough local area network (LAN) 1716. The chipset 1706 may includefunctionality for providing network connectivity through a networkinterface controller (NIC) 1722, such as a gigabit Ethernet adapter. ANIC 1722 may be capable of connecting the computing device 1700 to othercomputing nodes over a network 1716. It should be appreciated thatmultiple NICs 1722 may be present in the computing device 1700,connecting the computing device to other types of networks and remotecomputer systems. For example, the device 100 of FIG. 1 can be part of aspace object (e.g., satellite). The space object can be part of aconstellation network (e.g., or a space mesh network), such as a networkof space objects (e.g., orbiting around earth or other space object).The space object can comprise several devices 100, such as about 3 toabout 5 devices (e.g., configuring communication to multiple differentobjects in the constellation network).

The computing device 1700 may be connected to a mass storage device 1728that provides non-volatile storage for the computer. The mass storagedevice 1728 may store system programs, application programs, otherprogram modules, and data, which have been described in greater detailherein. The mass storage device 1728 may be connected to the computingdevice 1700 through a storage controller 1724 connected to the chipset1706. The mass storage device 1728 may consist of one or more physicalstorage units. A storage controller 1724 may interface with the physicalstorage units through a serial attached SCSI (SAS) interface, a serialadvanced technology attachment (SATA) interface, a fiber channel (FC)interface, or other type of interface for physically connecting andtransferring data between computers and physical storage units.

The computing device 1700 may store data on a mass storage device 1728by transforming the physical state of the physical storage units toreflect the information being stored. The specific transformation of aphysical state may depend on various factors and on differentimplementations of this description. Examples of such factors mayinclude, but are not limited to, the technology used to implement thephysical storage units and whether the mass storage device 1728 ischaracterized as primary or secondary storage and the like.

For example, the computing device 1700 may store information to the massstorage device 1728 by issuing instructions through a storage controller1724 to alter the magnetic characteristics of a particular locationwithin a magnetic disk drive unit, the reflective or refractivecharacteristics of a particular location in an optical storage unit, orthe electrical characteristics of a particular capacitor, transistor, orother discrete component in a solid-state storage unit. Othertransformations of physical media are possible without departing fromthe scope and spirit of the present description, with the foregoingexamples provided only to facilitate this description. The computingdevice 1700 may further read information from the mass storage device1728 by detecting the physical states or characteristics of one or moreparticular locations within the physical storage units.

In addition to the mass storage device 1728 described above, thecomputing device 500 may have access to other computer-readable storagemedia to store and retrieve information, such as program modules, datastructures, or other data. It should be appreciated by those skilled inthe art that computer-readable storage media may be any available mediathat provides for the storage of non-transitory data and that may beaccessed by the computing device 1700.

By way of example and not limitation, computer-readable storage mediamay include volatile and non-volatile, transitory computer-readablestorage media and non-transitory computer-readable storage media, andremovable and non-removable media implemented in any method ortechnology. Computer-readable storage media includes, but is not limitedto, RAM, ROM, erasable programmable ROM (“EPROM”), electrically erasableprogrammable ROM (“EEPROM”), flash memory or other solid-state memorytechnology, compact disc ROM (“CD-ROM”), digital versatile disk (“DVD”),high definition DVD (“HD-DVD”), BLU-RAY, or other optical storage,magnetic cassettes, magnetic tape, magnetic disk storage, other magneticstorage devices, or any other medium that may be used to store thedesired information in a non-transitory fashion.

A mass storage device, such as the mass storage device 1728 depicted inFIG. 17, may store an operating system utilized to control the operationof the computing device 1700. The operating system may comprise aversion of the LINUX operating system. The operating system may comprisea version of the WINDOWS SERVER operating system from the MICROSOFTCorporation. According to further aspects, the operating system maycomprise a version of the UNIX operating system. Various mobile phoneoperating systems, such as IOS and ANDROID, may also be utilized. Itshould be appreciated that other operating systems may also be utilized.The mass storage device 1728 may store other system or applicationprograms and data utilized by the computing device 1700.

The mass storage device 1728 or other computer-readable storage mediamay also be encoded with computer-executable instructions, which, whenloaded into the computing device 1700, transforms the computing devicefrom a general-purpose computing system into a special-purpose computercapable of implementing the aspects described herein. Thesecomputer-executable instructions transform the computing device 1700 byspecifying how the CPU(s) 1704 transition between states, as describedabove. The computing device 1700 may have access to computer-readablestorage media storing computer-executable instructions, which, whenexecuted by the computing device 1700, may perform the methods describedherein.

A computing device, such as the computing device 1700 depicted in FIG.17, may also include an input/output controller 1732 for receiving andprocessing input from a number of input devices, such as a keyboard, amouse, a touchpad, a touch screen, an electronic stylus, or other typeof input device. Similarly, an input/output controller 1732 may provideoutput to a display, such as a computer monitor, a flat-panel display, adigital projector, a printer, a plotter, or other type of output device.It will be appreciated that the computing device 1700 may not includeall of the components shown in FIG. 17, may include other componentsthat are not explicitly shown in FIG. 17, or may utilize an architecturecompletely different than that shown in FIG. 17.

As described herein, a computing device may be a physical computingdevice, such as the computing device 1700 of FIG. 17. A computing nodemay also include a virtual machine host process and one or more virtualmachine instances. Computer-executable instructions may be executed bythe physical hardware of a computing device indirectly throughinterpretation and/or execution of instructions stored and executed inthe context of a virtual machine.

It is to be understood that the methods and systems are not limited tospecific methods, specific components, or to particular implementations.It is also to be understood that the terminology used herein is for thepurpose of describing particular embodiments only and is not intended tobe limiting.

As used in the specification and the appended claims, the singular forms“a,” “an,” and “the” include plural referents unless the context clearlydictates otherwise. Ranges may be expressed herein as from “about” oneparticular value, and/or to “about” another particular value. When sucha range is expressed, another embodiment includes from the oneparticular value and/or to the other particular value. Similarly, whenvalues are expressed as approximations, by use of the antecedent“about,” it will be understood that the particular value forms anotherembodiment. It will be further understood that the endpoints of each ofthe ranges are significant both in relation to the other endpoint, andindependently of the other endpoint.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where said event or circumstance occurs and instances where itdoes not.

Throughout the description and claims of this specification, the word“comprise” and variations of the word, such as “comprising” and“comprises,” means “including but not limited to,” and is not intendedto exclude, for example, other components, integers or steps.“Exemplary” means “an example of” and is not intended to convey anindication of a preferred or ideal embodiment. “Such as” is not used ina restrictive sense, but for explanatory purposes.

Components are described that may be used to perform the describedmethods and systems. When combinations, subsets, interactions, groups,etc., of these components are described, it is understood that whilespecific references to each of the various individual and collectivecombinations and permutations of these may not be explicitly described,each is specifically contemplated and described herein, for all methodsand systems. This applies to all aspects of this application including,but not limited to, operations in described methods. Thus, if there area variety of additional operations that may be performed it isunderstood that each of these additional operations may be performedwith any specific embodiment or combination of embodiments of thedescribed methods.

As will be appreciated by one skilled in the art, the methods andsystems may take the form of an entirely hardware embodiment, anentirely software embodiment, or an embodiment combining software andhardware aspects. Furthermore, the methods and systems may take the formof a computer program product on a computer-readable storage mediumhaving computer-readable program instructions (e.g., computer software)embodied in the storage medium. More particularly, the present methodsand systems may take the form of web-implemented computer software. Anysuitable computer-readable storage medium may be utilized including harddisks, CD-ROMs, optical storage devices, or magnetic storage devices.

Embodiments of the methods and systems are described below withreference to block diagrams and flowchart illustrations of methods,systems, apparatuses and computer program products. It will beunderstood that each block of the block diagrams and flowchartillustrations, and combinations of blocks in the block diagrams andflowchart illustrations, respectively, may be implemented by computerprogram instructions. These computer program instructions may be loadedon a general-purpose computer, special-purpose computer, or otherprogrammable data processing apparatus to produce a machine, such thatthe instructions which execute on the computer or other programmabledata processing apparatus create a means for implementing the functionsspecified in the flowchart block or blocks.

These computer program instructions may also be stored in acomputer-readable memory that may direct a computer or otherprogrammable data processing apparatus to function in a particularmanner, such that the instructions stored in the computer-readablememory produce an article of manufacture including computer-readableinstructions for implementing the function specified in the flowchartblock or blocks. The computer program instructions may also be loadedonto a computer or other programmable data processing apparatus to causea series of operational steps to be performed on the computer or otherprogrammable apparatus to produce a computer-implemented process suchthat the instructions that execute on the computer or other programmableapparatus provide steps for implementing the functions specified in theflowchart block or blocks.

The various features and processes described above may be usedindependently of one another, or may be combined in various ways. Allpossible combinations and sub-combinations are intended to fall withinthe scope of this disclosure. In addition, certain methods or processblocks may be omitted in some implementations. The methods and processesdescribed herein are also not limited to any particular sequence, andthe blocks or states relating thereto may be performed in othersequences that are appropriate. For example, described blocks or statesmay be performed in an order other than that specifically described, ormultiple blocks or states may be combined in a single block or state.The example blocks or states may be performed in serial, in parallel, orin some other manner. Blocks or states may be added to or removed fromthe described example embodiments. The example systems and componentsdescribed herein may be configured differently than described. Forexample, elements may be added to, removed from, or rearranged comparedto the described example embodiments.

It will also be appreciated that various items are illustrated as beingstored in memory or on storage while being used, and that these items orportions thereof may be transferred between memory and other storagedevices for purposes of memory management and data integrity.Alternatively, in other embodiments, some or all of the software modulesand/or systems may execute in memory on another device and communicatewith the illustrated computing systems via inter-computer communication.Furthermore, in some embodiments, some or all of the systems and/ormodules may be implemented or provided in other ways, such as at leastpartially in firmware and/or hardware, including, but not limited to,one or more application-specific integrated circuits (“ASICs”), standardintegrated circuits, controllers (e.g., by executing appropriateinstructions, and including microcontrollers and/or embeddedcontrollers), field-programmable gate arrays (“FPGAs”), complexprogrammable logic devices (“CPLDs”), etc. Some or all of the modules,systems, and data structures may also be stored (e.g., as softwareinstructions or structured data) on a computer-readable medium, such asa hard disk, a memory, a network, or a portable media article to be readby an appropriate device or via an appropriate connection. The systems,modules, and data structures may also be transmitted as generated datasignals (e.g., as part of a carrier wave or other analog or digitalpropagated signal) on a variety of computer-readable transmission media,including wireless-based and wired/cable-based media, and may take avariety of forms (e.g., as part of a single or multiplexed analogsignal, or as multiple discrete digital packets or frames). Suchcomputer program products may also take other forms in otherembodiments. Accordingly, the present invention may be practiced withother computer system configurations.

While the methods and systems have been described in connection withpreferred embodiments and specific examples, it is not intended that thescope be limited to the particular embodiments set forth, as theembodiments herein are intended in all respects to be illustrativerather than restrictive.

It will be apparent to those skilled in the art that variousmodifications and variations may be made without departing from thescope or spirit of the present disclosure. Other embodiments will beapparent to those skilled in the art from consideration of thespecification and practices described herein. It is intended that thespecification and example figures be considered as exemplary only, witha true scope and spirit being indicated by the following claims.

1. A device for free space optical communication comprising: a defocuserconfigured to receive an optical signal from a laser and control a beamdivergence of the optical signal, wherein the optical signal comprises adata signal and a beacon signal; and a controller configured to causethe defocuser to adjust the beam divergence to a first beam divergencefor a first operational mode and a second beam divergence for a secondoperational mode, wherein the first operational mode comprises acommunication mode and the second operational mode comprises one or moreof a tracking mode or a link acquisition mode.
 2. The device of claim 1,wherein the defocuser comprises an adjustable lens configured to controlthe beam divergence.
 3. The device of claim 2, wherein the adjustablelens comprises a liquid lens configured to change shape, in response toan electrical signal, to modify the beam divergence.
 4. The device ofclaim 2, wherein the defocuser is configured to control the beamdivergence by adjusting a distance between the adjustable lens and asource of the optical signal.
 5. The device of claim 1, wherein thesecond beam divergence is wider than the first beam divergence. 6.(canceled)
 7. The device of claim 1, wherein the defocuser enables useof the laser and output optics for both the communication mode and oneor more of the tracking mode or the link acquisition mode.
 8. A methodfor free space optical communication comprising: adjusting, based on afirst operational mode, a defocuser from a first setting associated to afirst setting configured for a first beam divergence, wherein the firstoperational mode comprises a data communication mode; receiving, from alaser, a first optical signal; modifying, using the defocuser adjustedto the first setting, the first optical signal to have the first beamdivergence; adjusting, based on a second operational mode, the defocuserto a second setting configured for a second beam divergence, wherein thesecond operational mode comprise one or more of a tracking mode or alink acquisition mode; receiving, from the laser, a second opticalsignal; modifying, using the defocuser adjusted to the second setting,the second optical signal to have the second beam divergence; andoutputting one or more of the modified first optical signal or themodified second optical signal.
 9. The method of claim 8, wherein thedefocuser comprises an adjustable lens configured to control beamdivergence.
 10. The method of claim 9, wherein the adjustable lenscomprises a liquid lens configured to change shape, in response to anelectrical signal, to modify the beam divergence.
 11. The method ofclaim 9, wherein adjusting the defocuser to the first setting comprisesadjusting a distance between the adjustable lens and a source of theoptical signal to a first distance, and wherein adjusting the defocuserto the second setting comprises adjusting the distance between theadjustable lens and the source of the optical signal to a seconddistance different than the first distance.
 12. The method of claim 8,wherein the second beam divergence is wider than the first beamdivergence.
 13. (canceled)
 14. The method of claim 8, wherein thedefocuser enables use of the laser and output optics for both thecommunication mode and one or more of the tracking mode or the linkacquisition mode.
 15. A system for free space optical communicationcomprising: a first optical terminal; and a second optical terminalconfigured to communicate with the first optical terminal, wherein oneor more of the first optical terminal or the second optical terminalcomprises: a defocuser configured to receive an optical signal from alaser and control a beam divergence of the optical signal, wherein theoptical signal comprises a data signal and a beacon signal; and acontroller configured to cause the defocuser to adjust the beamdivergence to a first beam divergence for a first operational mode and asecond beam divergence for a second operational mode, wherein the firstoperational mode comprises a communication mode and the secondoperational mode comprises one or more of a tracking mode or a linkacquisition mode.
 16. The system of claim 15, wherein the defocusercomprises an adjustable lens configured to control the beam divergence.17. The system of claim 16, wherein the adjustable lens comprises aliquid lens configured to change shape, in response to an electricalsignal, to modify the beam divergence.
 18. The system of claim 16,wherein the defocuser is configured to control the beam divergence byadjusting a distance between the adjustable lens and a source of theoptical signal.
 19. The system of claim 15, wherein the second beamdivergence is wider than the first beam divergence.
 20. (canceled)